September 1, 2011

In things thick and thin: Cornell physicists explain how fluids — such as paint or paste - behave by observing how micron-sized suspended particles dance in real time. Using high-speed microscopy, the scientists unveil how these particles are responding to fluid flows from shear — a specific way of stirring. (Science, Sept. 2).

Observations by Xiang Cheng, Cornell post-doctoral researcher in physics and Itai Cohen, Cornell associate professor of physics, are the first to link direct imaging of the particle motions with changes in liquid viscosity.

Combining high-speed 3-D imaging techniques with a sensitive force-measuring device, the researchers tracked the motions of tiny particles suspended in the fluids while monitoring the thinning or thickening behaviors under shear.

They found that fluids become thinner when the particles – which normally move in a random way – get swept by the induced fluid flows.

In addition, they showed fluids became thicker or more viscous when particles were driven past one another too quickly for the fluid between them to drain or get out of the way. At such high speeds, the particles form clusters that lock together and make the fluid more viscous.

Grasping the physics of shear thinning and thickening isn't just good for at-home science experiments, knowledge of fluid phenomena are important for commerce. "In industry, understanding the thinning and thickening of materials is crucial for almost any transport process," Cohen said. These findings will improve the ability of scientists and engineers to handle complex fluids ranging from such industrial materials as paints, detergents and pastes, as well as such biological liquids as lymph and blood.

The researchers' observations refute theories that such changes in fluid viscosity result from the formation and destruction of particle layers under shear. The idea behind these theories is that, like lanes on a highway, streamlining particle trajectories reduces random collisions and enables particles to flow past each other more smoothly. When the particles form layers at low shear rates, the viscosity decreases, causing the fluid to thin; when the particle layers break up at high shear rates, the viscosity increases, causing the fluid to thicken.

However, by directly imaging the layering and measuring the fluid viscosity, the Cornell scientists found that while the amount of layering and delayering was comparable, the changes in viscosity were substantially different in the thinning and thickening regimes.

Moreover, the delayering occurred at shear rates much lower than those leading to thickening. Hence, they produced evidence that layering is not the major reason for viscosity changes in these suspensions.